protobuf/upb/hash/common.c

/*
 * Copyright (c) 2009-2021, Google LLC
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *     * Redistributions of source code must retain the above copyright
 *       notice, this list of conditions and the following disclaimer.
 *     * Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *     * Neither the name of Google LLC nor the
 *       names of its contributors may be used to endorse or promote products
 *       derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL Google LLC BE LIABLE FOR ANY DIRECT,
 * INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
 * LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
 * ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

/*
 * upb_table Implementation
 *
 * Implementation is heavily inspired by Lua's ltable.c.
 */

#include <string.h>

#include "upb/base/log2.h"
#include "upb/hash/int_table.h"
#include "upb/hash/str_table.h"

// Must be last.
#include "upb/port/def.inc"

#define UPB_MAXARRSIZE 16  // 2**16 = 64k.

// From Chromium.
#define ARRAY_SIZE(x) \
  ((sizeof(x) / sizeof(0 [x])) / ((size_t)(!(sizeof(x) % sizeof(0 [x])))))

static const double MAX_LOAD = 0.85;

/* The minimum utilization of the array part of a mixed hash/array table.  This
 * is a speed/memory-usage tradeoff (though it's not straightforward because of
 * cache effects).  The lower this is, the more memory we'll use. */
static const double MIN_DENSITY = 0.1;

static bool is_pow2(uint64_t v) { return v == 0 || (v & (v - 1)) == 0; }

static upb_value _upb_value_val(uint64_t val) {
  upb_value ret;
  _upb_value_setval(&ret, val);
  return ret;
}

static int log2ceil(uint64_t v) {
  int ret = 0;
  bool pow2 = is_pow2(v);
  while (v >>= 1) ret++;
  ret = pow2 ? ret : ret + 1;  // Ceiling.
  return UPB_MIN(UPB_MAXARRSIZE, ret);
}

char* upb_strdup2(const char* s, size_t len, upb_Arena* a) {
  size_t n;
  char* p;

  /* Prevent overflow errors. */
  if (len == SIZE_MAX) return NULL;
  /* Always null-terminate, even if binary data; but don't rely on the input to
   * have a null-terminating byte since it may be a raw binary buffer. */
  n = len + 1;
  p = upb_Arena_Malloc(a, n);
  if (p) {
    if (len != 0) memcpy(p, s, len);
    p[len] = 0;
  }
  return p;
}

/* A type to represent the lookup key of either a strtable or an inttable. */
typedef union {
  uintptr_t num;
  struct {
    const char* str;
    size_t len;
  } str;
} lookupkey_t;

static lookupkey_t strkey2(const char* str, size_t len) {
  lookupkey_t k;
  k.str.str = str;
  k.str.len = len;
  return k;
}

static lookupkey_t intkey(uintptr_t key) {
  lookupkey_t k;
  k.num = key;
  return k;
}

typedef uint32_t hashfunc_t(upb_tabkey key);
typedef bool eqlfunc_t(upb_tabkey k1, lookupkey_t k2);

/* Base table (shared code) ***************************************************/

static uint32_t upb_inthash(uintptr_t key) { return (uint32_t)key; }

static const upb_tabent* upb_getentry(const upb_table* t, uint32_t hash) {
  return t->entries + (hash & t->mask);
}

static bool upb_arrhas(upb_tabval key) { return key.val != (uint64_t)-1; }

static bool isfull(upb_table* t) { return t->count == t->max_count; }

static bool init(upb_table* t, uint8_t size_lg2, upb_Arena* a) {
  size_t bytes;

  t->count = 0;
  t->size_lg2 = size_lg2;
  t->mask = upb_table_size(t) ? upb_table_size(t) - 1 : 0;
  t->max_count = upb_table_size(t) * MAX_LOAD;
  bytes = upb_table_size(t) * sizeof(upb_tabent);
  if (bytes > 0) {
    t->entries = upb_Arena_Malloc(a, bytes);
    if (!t->entries) return false;
    memset(t->entries, 0, bytes);
  } else {
    t->entries = NULL;
  }
  return true;
}

static upb_tabent* emptyent(upb_table* t, upb_tabent* e) {
  upb_tabent* begin = t->entries;
  upb_tabent* end = begin + upb_table_size(t);
  for (e = e + 1; e < end; e++) {
    if (upb_tabent_isempty(e)) return e;
  }
  for (e = begin; e < end; e++) {
    if (upb_tabent_isempty(e)) return e;
  }
  UPB_ASSERT(false);
  return NULL;
}

static upb_tabent* getentry_mutable(upb_table* t, uint32_t hash) {
  return (upb_tabent*)upb_getentry(t, hash);
}

static const upb_tabent* findentry(const upb_table* t, lookupkey_t key,
                                   uint32_t hash, eqlfunc_t* eql) {
  const upb_tabent* e;

  if (t->size_lg2 == 0) return NULL;
  e = upb_getentry(t, hash);
  if (upb_tabent_isempty(e)) return NULL;
  while (1) {
    if (eql(e->key, key)) return e;
    if ((e = e->next) == NULL) return NULL;
  }
}

static upb_tabent* findentry_mutable(upb_table* t, lookupkey_t key,
                                     uint32_t hash, eqlfunc_t* eql) {
  return (upb_tabent*)findentry(t, key, hash, eql);
}

static bool lookup(const upb_table* t, lookupkey_t key, upb_value* v,
                   uint32_t hash, eqlfunc_t* eql) {
  const upb_tabent* e = findentry(t, key, hash, eql);
  if (e) {
    if (v) {
      _upb_value_setval(v, e->val.val);
    }
    return true;
  } else {
    return false;
  }
}

/* The given key must not already exist in the table. */
static void insert(upb_table* t, lookupkey_t key, upb_tabkey tabkey,
                   upb_value val, uint32_t hash, hashfunc_t* hashfunc,
                   eqlfunc_t* eql) {
  upb_tabent* mainpos_e;
  upb_tabent* our_e;

  UPB_ASSERT(findentry(t, key, hash, eql) == NULL);

  t->count++;
  mainpos_e = getentry_mutable(t, hash);
  our_e = mainpos_e;

  if (upb_tabent_isempty(mainpos_e)) {
    /* Our main position is empty; use it. */
    our_e->next = NULL;
  } else {
    /* Collision. */
    upb_tabent* new_e = emptyent(t, mainpos_e);
    /* Head of collider's chain. */
    upb_tabent* chain = getentry_mutable(t, hashfunc(mainpos_e->key));
    if (chain == mainpos_e) {
      /* Existing ent is in its main position (it has the same hash as us, and
       * is the head of our chain).  Insert to new ent and append to this chain.
       */
      new_e->next = mainpos_e->next;
      mainpos_e->next = new_e;
      our_e = new_e;
    } else {
      /* Existing ent is not in its main position (it is a node in some other
       * chain).  This implies that no existing ent in the table has our hash.
       * Evict it (updating its chain) and use its ent for head of our chain. */
      *new_e = *mainpos_e; /* copies next. */
      while (chain->next != mainpos_e) {
        chain = (upb_tabent*)chain->next;
        UPB_ASSERT(chain);
      }
      chain->next = new_e;
      our_e = mainpos_e;
      our_e->next = NULL;
    }
  }
  our_e->key = tabkey;
  our_e->val.val = val.val;
  UPB_ASSERT(findentry(t, key, hash, eql) == our_e);
}

static bool rm(upb_table* t, lookupkey_t key, upb_value* val,
               upb_tabkey* removed, uint32_t hash, eqlfunc_t* eql) {
  upb_tabent* chain = getentry_mutable(t, hash);
  if (upb_tabent_isempty(chain)) return false;
  if (eql(chain->key, key)) {
    /* Element to remove is at the head of its chain. */
    t->count--;
    if (val) _upb_value_setval(val, chain->val.val);
    if (removed) *removed = chain->key;
    if (chain->next) {
      upb_tabent* move = (upb_tabent*)chain->next;
      *chain = *move;
      move->key = 0; /* Make the slot empty. */
    } else {
      chain->key = 0; /* Make the slot empty. */
    }
    return true;
  } else {
    /* Element to remove is either in a non-head position or not in the
     * table. */
    while (chain->next && !eql(chain->next->key, key)) {
      chain = (upb_tabent*)chain->next;
    }
    if (chain->next) {
      /* Found element to remove. */
      upb_tabent* rm = (upb_tabent*)chain->next;
      t->count--;
      if (val) _upb_value_setval(val, chain->next->val.val);
      if (removed) *removed = rm->key;
      rm->key = 0; /* Make the slot empty. */
      chain->next = rm->next;
      return true;
    } else {
      /* Element to remove is not in the table. */
      return false;
    }
  }
}

static size_t next(const upb_table* t, size_t i) {
  do {
    if (++i >= upb_table_size(t)) return SIZE_MAX - 1; /* Distinct from -1. */
  } while (upb_tabent_isempty(&t->entries[i]));

  return i;
}

static size_t begin(const upb_table* t) { return next(t, -1); }

/* upb_strtable ***************************************************************/

/* A simple "subclass" of upb_table that only adds a hash function for strings.
 */

static upb_tabkey strcopy(lookupkey_t k2, upb_Arena* a) {
  uint32_t len = (uint32_t)k2.str.len;
  char* str = upb_Arena_Malloc(a, k2.str.len + sizeof(uint32_t) + 1);
  if (str == NULL) return 0;
  memcpy(str, &len, sizeof(uint32_t));
  if (k2.str.len) memcpy(str + sizeof(uint32_t), k2.str.str, k2.str.len);
  str[sizeof(uint32_t) + k2.str.len] = '\0';
  return (uintptr_t)str;
}

/* Adapted from ABSL's wyhash. */

static uint64_t UnalignedLoad64(const void* p) {
  uint64_t val;
  memcpy(&val, p, 8);
  return val;
}

static uint32_t UnalignedLoad32(const void* p) {
  uint32_t val;
  memcpy(&val, p, 4);
  return val;
}

#if defined(_MSC_VER) && defined(_M_X64)
#include <intrin.h>
#endif

/* Computes a * b, returning the low 64 bits of the result and storing the high
 * 64 bits in |*high|. */
static uint64_t upb_umul128(uint64_t v0, uint64_t v1, uint64_t* out_high) {
#ifdef __SIZEOF_INT128__
  __uint128_t p = v0;
  p *= v1;
  *out_high = (uint64_t)(p >> 64);
  return (uint64_t)p;
#elif defined(_MSC_VER) && defined(_M_X64)
  return _umul128(v0, v1, out_high);
#else
  uint64_t a32 = v0 >> 32;
  uint64_t a00 = v0 & 0xffffffff;
  uint64_t b32 = v1 >> 32;
  uint64_t b00 = v1 & 0xffffffff;
  uint64_t high = a32 * b32;
  uint64_t low = a00 * b00;
  uint64_t mid1 = a32 * b00;
  uint64_t mid2 = a00 * b32;
  low += (mid1 << 32) + (mid2 << 32);
  // Omit carry bit, for mixing we do not care about exact numerical precision.
  high += (mid1 >> 32) + (mid2 >> 32);
  *out_high = high;
  return low;
#endif
}

static uint64_t WyhashMix(uint64_t v0, uint64_t v1) {
  uint64_t high;
  uint64_t low = upb_umul128(v0, v1, &high);
  return low ^ high;
}

static uint64_t Wyhash(const void* data, size_t len, uint64_t seed,
                       const uint64_t salt[]) {
  const uint8_t* ptr = (const uint8_t*)data;
  uint64_t starting_length = (uint64_t)len;
  uint64_t current_state = seed ^ salt[0];

  if (len > 64) {
    // If we have more than 64 bytes, we're going to handle chunks of 64
    // bytes at a time. We're going to build up two separate hash states
    // which we will then hash together.
    uint64_t duplicated_state = current_state;

    do {
      uint64_t a = UnalignedLoad64(ptr);
      uint64_t b = UnalignedLoad64(ptr + 8);
      uint64_t c = UnalignedLoad64(ptr + 16);
      uint64_t d = UnalignedLoad64(ptr + 24);
      uint64_t e = UnalignedLoad64(ptr + 32);
      uint64_t f = UnalignedLoad64(ptr + 40);
      uint64_t g = UnalignedLoad64(ptr + 48);
      uint64_t h = UnalignedLoad64(ptr + 56);

      uint64_t cs0 = WyhashMix(a ^ salt[1], b ^ current_state);
      uint64_t cs1 = WyhashMix(c ^ salt[2], d ^ current_state);
      current_state = (cs0 ^ cs1);

      uint64_t ds0 = WyhashMix(e ^ salt[3], f ^ duplicated_state);
      uint64_t ds1 = WyhashMix(g ^ salt[4], h ^ duplicated_state);
      duplicated_state = (ds0 ^ ds1);

      ptr += 64;
      len -= 64;
    } while (len > 64);

    current_state = current_state ^ duplicated_state;
  }

  // We now have a data `ptr` with at most 64 bytes and the current state
  // of the hashing state machine stored in current_state.
  while (len > 16) {
    uint64_t a = UnalignedLoad64(ptr);
    uint64_t b = UnalignedLoad64(ptr + 8);

    current_state = WyhashMix(a ^ salt[1], b ^ current_state);

    ptr += 16;
    len -= 16;
  }

  // We now have a data `ptr` with at most 16 bytes.
  uint64_t a = 0;
  uint64_t b = 0;
  if (len > 8) {
    // When we have at least 9 and at most 16 bytes, set A to the first 64
    // bits of the input and B to the last 64 bits of the input. Yes, they will
    // overlap in the middle if we are working with less than the full 16
    // bytes.
    a = UnalignedLoad64(ptr);
    b = UnalignedLoad64(ptr + len - 8);
  } else if (len > 3) {
    // If we have at least 4 and at most 8 bytes, set A to the first 32
    // bits and B to the last 32 bits.
    a = UnalignedLoad32(ptr);
    b = UnalignedLoad32(ptr + len - 4);
  } else if (len > 0) {
    // If we have at least 1 and at most 3 bytes, read all of the provided
    // bits into A, with some adjustments.
    a = ((ptr[0] << 16) | (ptr[len >> 1] << 8) | ptr[len - 1]);
    b = 0;
  } else {
    a = 0;
    b = 0;
  }

  uint64_t w = WyhashMix(a ^ salt[1], b ^ current_state);
  uint64_t z = salt[1] ^ starting_length;
  return WyhashMix(w, z);
}

const uint64_t kWyhashSalt[5] = {
    0x243F6A8885A308D3ULL, 0x13198A2E03707344ULL, 0xA4093822299F31D0ULL,
    0x082EFA98EC4E6C89ULL, 0x452821E638D01377ULL,
};

uint32_t _upb_Hash(const void* p, size_t n, uint64_t seed) {
  return Wyhash(p, n, seed, kWyhashSalt);
}

static uint32_t _upb_Hash_NoSeed(const char* p, size_t n) {
  return _upb_Hash(p, n, 0);
}

static uint32_t strhash(upb_tabkey key) {
  uint32_t len;
  char* str = upb_tabstr(key, &len);
  return _upb_Hash_NoSeed(str, len);
}

static bool streql(upb_tabkey k1, lookupkey_t k2) {
  uint32_t len;
  char* str = upb_tabstr(k1, &len);
  return len == k2.str.len && (len == 0 || memcmp(str, k2.str.str, len) == 0);
}

bool upb_strtable_init(upb_strtable* t, size_t expected_size, upb_Arena* a) {
  // Multiply by approximate reciprocal of MAX_LOAD (0.85), with pow2
  // denominator.
  size_t need_entries = (expected_size + 1) * 1204 / 1024;
  UPB_ASSERT(need_entries >= expected_size * 0.85);
  int size_lg2 = upb_Log2Ceiling(need_entries);
  return init(&t->t, size_lg2, a);
}

void upb_strtable_clear(upb_strtable* t) {
  size_t bytes = upb_table_size(&t->t) * sizeof(upb_tabent);
  t->t.count = 0;
  memset((char*)t->t.entries, 0, bytes);
}

bool upb_strtable_resize(upb_strtable* t, size_t size_lg2, upb_Arena* a) {
  upb_strtable new_table;
  if (!init(&new_table.t, size_lg2, a)) return false;

  intptr_t iter = UPB_STRTABLE_BEGIN;
  upb_StringView key;
  upb_value val;
  while (upb_strtable_next2(t, &key, &val, &iter)) {
    upb_strtable_insert(&new_table, key.data, key.size, val, a);
  }
  *t = new_table;
  return true;
}

bool upb_strtable_insert(upb_strtable* t, const char* k, size_t len,
                         upb_value v, upb_Arena* a) {
  lookupkey_t key;
  upb_tabkey tabkey;
  uint32_t hash;

  if (isfull(&t->t)) {
    /* Need to resize.  New table of double the size, add old elements to it. */
    if (!upb_strtable_resize(t, t->t.size_lg2 + 1, a)) {
      return false;
    }
  }

  key = strkey2(k, len);
  tabkey = strcopy(key, a);
  if (tabkey == 0) return false;

  hash = _upb_Hash_NoSeed(key.str.str, key.str.len);
  insert(&t->t, key, tabkey, v, hash, &strhash, &streql);
  return true;
}

bool upb_strtable_lookup2(const upb_strtable* t, const char* key, size_t len,
                          upb_value* v) {
  uint32_t hash = _upb_Hash_NoSeed(key, len);
  return lookup(&t->t, strkey2(key, len), v, hash, &streql);
}

bool upb_strtable_remove2(upb_strtable* t, const char* key, size_t len,
                          upb_value* val) {
  uint32_t hash = _upb_Hash_NoSeed(key, len);
  upb_tabkey tabkey;
  return rm(&t->t, strkey2(key, len), val, &tabkey, hash, &streql);
}

/* Iteration */

void upb_strtable_begin(upb_strtable_iter* i, const upb_strtable* t) {
  i->t = t;
  i->index = begin(&t->t);
}

void upb_strtable_next(upb_strtable_iter* i) {
  i->index = next(&i->t->t, i->index);
}

bool upb_strtable_done(const upb_strtable_iter* i) {
  if (!i->t) return true;
  return i->index >= upb_table_size(&i->t->t) ||
         upb_tabent_isempty(str_tabent(i));
}

upb_StringView upb_strtable_iter_key(const upb_strtable_iter* i) {
  upb_StringView key;
  uint32_t len;
  UPB_ASSERT(!upb_strtable_done(i));
  key.data = upb_tabstr(str_tabent(i)->key, &len);
  key.size = len;
  return key;
}

upb_value upb_strtable_iter_value(const upb_strtable_iter* i) {
  UPB_ASSERT(!upb_strtable_done(i));
  return _upb_value_val(str_tabent(i)->val.val);
}

void upb_strtable_iter_setdone(upb_strtable_iter* i) {
  i->t = NULL;
  i->index = SIZE_MAX;
}

bool upb_strtable_iter_isequal(const upb_strtable_iter* i1,
                               const upb_strtable_iter* i2) {
  if (upb_strtable_done(i1) && upb_strtable_done(i2)) return true;
  return i1->t == i2->t && i1->index == i2->index;
}

/* upb_inttable ***************************************************************/

/* For inttables we use a hybrid structure where small keys are kept in an
 * array and large keys are put in the hash table. */

static uint32_t inthash(upb_tabkey key) { return upb_inthash(key); }

static bool inteql(upb_tabkey k1, lookupkey_t k2) { return k1 == k2.num; }

static upb_tabval* mutable_array(upb_inttable* t) {
  return (upb_tabval*)t->array;
}

static upb_tabval* inttable_val(upb_inttable* t, uintptr_t key) {
  if (key < t->array_size) {
    return upb_arrhas(t->array[key]) ? &(mutable_array(t)[key]) : NULL;
  } else {
    upb_tabent* e =
        findentry_mutable(&t->t, intkey(key), upb_inthash(key), &inteql);
    return e ? &e->val : NULL;
  }
}

static const upb_tabval* inttable_val_const(const upb_inttable* t,
                                            uintptr_t key) {
  return inttable_val((upb_inttable*)t, key);
}

size_t upb_inttable_count(const upb_inttable* t) {
  return t->t.count + t->array_count;
}

static void check(upb_inttable* t) {
  UPB_UNUSED(t);
#if defined(UPB_DEBUG_TABLE) && !defined(NDEBUG)
  {
    // This check is very expensive (makes inserts/deletes O(N)).
    size_t count = 0;
    intptr_t iter = UPB_INTTABLE_BEGIN;
    uintptr_t key;
    upb_value val;
    while (upb_inttable_next(t, &key, &val, &iter)) {
      UPB_ASSERT(upb_inttable_lookup(t, key, NULL));
    }
    UPB_ASSERT(count == upb_inttable_count(t));
  }
#endif
}

bool upb_inttable_sizedinit(upb_inttable* t, size_t asize, int hsize_lg2,
                            upb_Arena* a) {
  size_t array_bytes;

  if (!init(&t->t, hsize_lg2, a)) return false;
  /* Always make the array part at least 1 long, so that we know key 0
   * won't be in the hash part, which simplifies things. */
  t->array_size = UPB_MAX(1, asize);
  t->array_count = 0;
  array_bytes = t->array_size * sizeof(upb_value);
  t->array = upb_Arena_Malloc(a, array_bytes);
  if (!t->array) {
    return false;
  }
  memset(mutable_array(t), 0xff, array_bytes);
  check(t);
  return true;
}

bool upb_inttable_init(upb_inttable* t, upb_Arena* a) {
  return upb_inttable_sizedinit(t, 0, 4, a);
}

bool upb_inttable_insert(upb_inttable* t, uintptr_t key, upb_value val,
                         upb_Arena* a) {
  upb_tabval tabval;
  tabval.val = val.val;
  UPB_ASSERT(
      upb_arrhas(tabval)); /* This will reject (uint64_t)-1.  Fix this. */

  if (key < t->array_size) {
    UPB_ASSERT(!upb_arrhas(t->array[key]));
    t->array_count++;
    mutable_array(t)[key].val = val.val;
  } else {
    if (isfull(&t->t)) {
      /* Need to resize the hash part, but we re-use the array part. */
      size_t i;
      upb_table new_table;

      if (!init(&new_table, t->t.size_lg2 + 1, a)) {
        return false;
      }

      for (i = begin(&t->t); i < upb_table_size(&t->t); i = next(&t->t, i)) {
        const upb_tabent* e = &t->t.entries[i];
        uint32_t hash;
        upb_value v;

        _upb_value_setval(&v, e->val.val);
        hash = upb_inthash(e->key);
        insert(&new_table, intkey(e->key), e->key, v, hash, &inthash, &inteql);
      }

      UPB_ASSERT(t->t.count == new_table.count);

      t->t = new_table;
    }
    insert(&t->t, intkey(key), key, val, upb_inthash(key), &inthash, &inteql);
  }
  check(t);
  return true;
}

bool upb_inttable_lookup(const upb_inttable* t, uintptr_t key, upb_value* v) {
  const upb_tabval* table_v = inttable_val_const(t, key);
  if (!table_v) return false;
  if (v) _upb_value_setval(v, table_v->val);
  return true;
}

bool upb_inttable_replace(upb_inttable* t, uintptr_t key, upb_value val) {
  upb_tabval* table_v = inttable_val(t, key);
  if (!table_v) return false;
  table_v->val = val.val;
  return true;
}

bool upb_inttable_remove(upb_inttable* t, uintptr_t key, upb_value* val) {
  bool success;
  if (key < t->array_size) {
    if (upb_arrhas(t->array[key])) {
      upb_tabval empty = UPB_TABVALUE_EMPTY_INIT;
      t->array_count--;
      if (val) {
        _upb_value_setval(val, t->array[key].val);
      }
      mutable_array(t)[key] = empty;
      success = true;
    } else {
      success = false;
    }
  } else {
    success = rm(&t->t, intkey(key), val, NULL, upb_inthash(key), &inteql);
  }
  check(t);
  return success;
}

void upb_inttable_compact(upb_inttable* t, upb_Arena* a) {
  /* A power-of-two histogram of the table keys. */
  size_t counts[UPB_MAXARRSIZE + 1] = {0};

  /* The max key in each bucket. */
  uintptr_t max[UPB_MAXARRSIZE + 1] = {0};

  {
    intptr_t iter = UPB_INTTABLE_BEGIN;
    uintptr_t key;
    upb_value val;
    while (upb_inttable_next(t, &key, &val, &iter)) {
      int bucket = log2ceil(key);
      max[bucket] = UPB_MAX(max[bucket], key);
      counts[bucket]++;
    }
  }

  /* Find the largest power of two that satisfies the MIN_DENSITY
   * definition (while actually having some keys). */
  size_t arr_count = upb_inttable_count(t);
  int size_lg2;
  upb_inttable new_t;

  for (size_lg2 = ARRAY_SIZE(counts) - 1; size_lg2 > 0; size_lg2--) {
    if (counts[size_lg2] == 0) {
      /* We can halve again without losing any entries. */
      continue;
    } else if (arr_count >= (1 << size_lg2) * MIN_DENSITY) {
      break;
    }

    arr_count -= counts[size_lg2];
  }

  UPB_ASSERT(arr_count <= upb_inttable_count(t));

  {
    /* Insert all elements into new, perfectly-sized table. */
    size_t arr_size = max[size_lg2] + 1; /* +1 so arr[max] will fit. */
    size_t hash_count = upb_inttable_count(t) - arr_count;
    size_t hash_size = hash_count ? (hash_count / MAX_LOAD) + 1 : 0;
    int hashsize_lg2 = log2ceil(hash_size);

    upb_inttable_sizedinit(&new_t, arr_size, hashsize_lg2, a);

    {
      intptr_t iter = UPB_INTTABLE_BEGIN;
      uintptr_t key;
      upb_value val;
      while (upb_inttable_next(t, &key, &val, &iter)) {
        upb_inttable_insert(&new_t, key, val, a);
      }
    }

    UPB_ASSERT(new_t.array_size == arr_size);
    UPB_ASSERT(new_t.t.size_lg2 == hashsize_lg2);
  }
  *t = new_t;
}

// Iteration.

bool upb_inttable_next(const upb_inttable* t, uintptr_t* key, upb_value* val,
                       intptr_t* iter) {
  intptr_t i = *iter;
  if ((size_t)(i + 1) <= t->array_size) {
    while ((size_t)++i < t->array_size) {
      upb_tabval ent = t->array[i];
      if (upb_arrhas(ent)) {
        *key = i;
        *val = _upb_value_val(ent.val);
        *iter = i;
        return true;
      }
    }
    i--;  // Back up to exactly one position before the start of the table.
  }

  size_t tab_idx = next(&t->t, i - t->array_size);
  if (tab_idx < upb_table_size(&t->t)) {
    upb_tabent* ent = &t->t.entries[tab_idx];
    *key = ent->key;
    *val = _upb_value_val(ent->val.val);
    *iter = tab_idx + t->array_size;
    return true;
  }

  return false;
}

void upb_inttable_removeiter(upb_inttable* t, intptr_t* iter) {
  intptr_t i = *iter;
  if ((size_t)i < t->array_size) {
    t->array_count--;
    mutable_array(t)[i].val = -1;
  } else {
    upb_tabent* ent = &t->t.entries[i - t->array_size];
    upb_tabent* prev = NULL;

    // Linear search, not great.
    upb_tabent* end = &t->t.entries[upb_table_size(&t->t)];
    for (upb_tabent* e = t->t.entries; e != end; e++) {
      if (e->next == ent) {
        prev = e;
        break;
      }
    }

    if (prev) {
      prev->next = ent->next;
    }

    t->t.count--;
    ent->key = 0;
    ent->next = NULL;
  }
}

bool upb_strtable_next2(const upb_strtable* t, upb_StringView* key,
                        upb_value* val, intptr_t* iter) {
  size_t tab_idx = next(&t->t, *iter);
  if (tab_idx < upb_table_size(&t->t)) {
    upb_tabent* ent = &t->t.entries[tab_idx];
    uint32_t len;
    key->data = upb_tabstr(ent->key, &len);
    key->size = len;
    *val = _upb_value_val(ent->val.val);
    *iter = tab_idx;
    return true;
  }

  return false;
}

void upb_strtable_removeiter(upb_strtable* t, intptr_t* iter) {
  intptr_t i = *iter;
  upb_tabent* ent = &t->t.entries[i];
  upb_tabent* prev = NULL;

  // Linear search, not great.
  upb_tabent* end = &t->t.entries[upb_table_size(&t->t)];
  for (upb_tabent* e = t->t.entries; e != end; e++) {
    if (e->next == ent) {
      prev = e;
      break;
    }
  }

  if (prev) {
    prev->next = ent->next;
  }

  t->t.count--;
  ent->key = 0;
  ent->next = NULL;
}